This is a more crude and unexacting method I've heard of and used. It seems to work in that my arrows don't linger on sloped shots.

Tie a plumb bob or something weighted from your ceiling with a string that runs to the floor. This will represent true vertical. Now kneel on the floor in front of the string with your bow and draw as you would (with an arrow of course - don't want to dry fire it!) Aim so that the top pin is on the string and level the bow using the sight level. The pins below the top pin should follow the string as closely as possible. If not, adjust the 3rd axis accordingly.

Hope it helps......Fritz